ecs455: chapter 4 - 4 - 1 - multiple access - 1… · multiple access techniques 18 yallow many...

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ECS455: Chapter 4 Multiple Access

Asst. Prof. Dr. Prapun [email protected]

Office Hours: BKD, 6th floor of Sirindhralai building

Tuesday 14:20-15:20Wednesday 14:20-15:20Friday 9:15-10:15

Dr.Prapun Suksompongprapun.com/ecs455

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ECS455: Chapter 4 Multiple Access

4.1 Duplexing: TDD and FDD




Duplexing

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Allow the subscriber to send “simultaneously” information to the base station while receiving information from the base station.

Talk and listen simultaneously.

Definitions:Forward channel or downlink (DL) is usedfor communication from the infrastructure to the users/stationsReverse channel or uplink (UL) is used forcommunication from users/stations back to the infrastructure.

Two techniques1. Frequency division duplexing (FDD)2. Time division duplexing (TDD)

[Rappaport, 2002, Ch 9]

Frequency Division Duplexing (FDD)

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Provide two distinct bands of frequencies (simplex channels) for every user.

The forward band provides traffic from the base station to the mobile.

The reverse band provides traffic from the mobile to the base station.

Any duplex channel actually consists of two simplex channels (a forward and reverse).

Most commercial cellular systems are based on FDD.

FDD Examples

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[Karim and Sarraf, 2002, Fig 6-1]

[Karim and Sarraf, 2002, Fig 5-1]

GSM

UMTS

Power of Sinc Function

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G f T f

[dB]

G f T f

G f T f

Problems of FDD

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Each transceiver simultaneously transmits and receives radio signals

The signals transmitted and received can vary by more than 100 dB. The signals in each direction need to occupy bands that are separated far apart (tens of MHz)

A device called a duplexer is required to filter out any interference between the two bands.

[Tse and Viswanath, 2005, Ch 4, p 121]

Time Division Duplexing (TDD)

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The UL and DL data are transmitted on the same carrier frequency at different times. (Taking turns)

Use time instead of frequency to provide both forward and reverse links.Each duplex channel has both a forward time slot and a reverse time slot.

“Unpaired spectrum”

Time Division Duplexing (TDD)

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If the time separation between the forward and reverse time slot is small, then the transmission and reception of data appears simultaneous to the users at both the subscriber unit and on the base station side.

Used in Bluetooth and Mobile WiMAX

LTE can be FDD or TDD.

FDD and TDD LTE frequency bands

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TDD LTE frequency band allocationsFDD LTE frequency band allocations

[http://www.radio-electronics.com/info/cellulartelecomms/lte-long-term-evolution/lte-frequency-spectrum.php]

LTE TDD has been commercial since 2011 and is gaining global momentum. The initial global unpaired bands include

2.3GHz (B40) used in India and 2.6 GHz (B38) used Europe, with variations (B41) in the U.S. and Japan.

China is expected to launch LTE TDD in multiple global bands.

Global LTE TDD spectrum

11 [Qualcomm, “LTE TDD - the global solution for unpaired spectrum”, September, 2014]

Operating bands specified for LTE in 3GPP above 1 GHz

12 [Dahlman, Parkvall, and Skold, 2016]

Advantages of FDD

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TDD frames need to incorporate guard periods equal to the max round trip propagation delay to avoid interference between uplink and downlink under worst-case conditions.

There is a time latency created by TDD due to the fact that communications is not full duplex in the truest sense.

Disadvantages of TDD

Advantages of TDD

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Duplexer is not required.Enable adjustment of the downlink/uplink ratio to efficiently support asymmetric DL/UL traffic.

With FDD, DL and UL have fixed and generally, equal DL and UL bandwidths.

Ability to implement in unpaired spectrumFDD requires a pair of channelsTDD only requires a single channel for both DL and UL providing greater flexibility for adaptation to varied global spectrum allocations.

Assure channel reciprocity for better support of link adaptation, MIMO and other closed loop advanced antenna technologies.

Disadvantages of FDD

Channel Reciprocity

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Usually, for better performance, to choose the coding/modulation scheme and its parameters, the transmitter needs to learn the channel state information (CSI). It is relatively easier for a receiver to find CSI.

Is the decoded message readable or gibberish?Usually, this is done by the transmitter sending a preamble training sequences or pilot symbols to the receiver.

How can a transmitter obtain CSI?The corresponding receiver may convey this information via a feedback link.

An overhead which reduce the efficiency of the system.Even worse when there are many parameters of the channel to learn. o For example, for MIMO, there are many antennas.

The information can only be used for a short time duration.The channel changes due to mobility of the Tx, Rx, or objects in the environment.

Use channel reciprocity

Channel Reciprocity

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The channel from point A to point B is identical to the channel from B to A if the channel is measured at the same time and same frequency.

The channel from A to B can be estimated at A using the pilot symbols embedded in the signal sent from B.

Using the reciprocity principle, this estimate is also an estimate for the channel from A to B.

In FDD systems, the two directions use different frequencies. Thus, channel reciprocity does not hold.

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Chapter 4Multiple Access

4.2 FDMA and TDMA




Multiple Access Techniques

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Allow many mobile users to share simultaneously a finite amount of radio spectrum. For high quality communications, this must be done without severe degradation in the performance of the system.Important access techniques1. Frequency division multiple access (FDMA)2. Time division multiple access (TDMA) 3. Spread spectrum multiple access (SSMA)

Frequency Hopped Multiple Access (FHMA)Code division multiple access (CDMA)

4. Space division multiple access (SDMA)5. Random access

ALOHA

Frequency division multiple access (FDMA)

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The oldest multiple access scheme for wireless communications.

Used exclusively for multiple access in 1G down to individual resource units or physical channels.

Assign individual channels to individual users.Different carrier frequency is assigned to each user so that the resulting spectra do not overlap.During the period of the call, no other user can share the same channel.

FDMA (2)

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Band-pass filtering (or heterodyning) enables separate demodulation of each channel.

If an FDMA channel is not in use, then it sits idle and cannotbe used by other users to increase or share capacity.

It is essentially a wasted resource.

In FDD systems, the users are assigned a channel as a pair of frequencies.

[Rappaport, 2002, Ch 9, p. 449]

Time division multiple access (TDMA)

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Divide the radio spectrum into time slots.In each slot only one user is allowed to either transmit or receive.A channel may be thought of as a particular time slot that reoccurs every frame, where N time slots comprise a frame.

Transmit data in a buffer-and-burst methodThe transmission for any user is non-continuous.Digital data and digital modulation must be used with TDMA.This results in low battery consumption, since the subscriber transmitter can be turned off when not in use (which is most of the time).

An obvious choice in the 1980s for digital mobile communications.

User 1 User 2 User 3 User 1 User 2 User 3

a framet

FDMA vs. TDMA

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Tradeoffs

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TDMA transmissions are slottedRequire the receivers to be synchronized for each data burst.Guard times are necessary to separate users. This results in larger overheads.FDMA allows completely uncoordinated transmission in the time domain

No time synchronization among users is required.

The complexity of FDMA mobile systems is lower when compared to TDMA systems, though this is changing as digital signal processing methods improve for TDMA.Since FDMA is a continuous transmission scheme, fewer bits are needed for overhead purposes (such as synchronization and framing bits) as compared to TDMA.FDMA needs to use costly bandpass filters.

For TDMA, no filters are required to separate individual physical channels.

Guard Band vs. Guard Time

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FDMA TDMA

Example: GSM

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GSM utilizes a combination of FDMA and TDMATwo-dimensional channel structure

[Figure 7.2, Heine, 1998]

FDMA

GSM FDMA/TDMA with one active time slot

Each narrowband channel has bandwidth 200 kHz.Time is divided into slots of length T =577 s.

The FDMA/TDMA structure of GSM

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In full-rate configuration, eight time slots (TSs) are mapped on every frequency.

[Figure 7.1, Heine, 1998]

A BS with 6 carriers, as shown here, has 48 (8 times 6)physical channels (in fullrate configuration).

Classifications of Medium Access Control (MAC)

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Multiple Access Techniques in Cellular System

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Cellular System Multiple Access Technique

Advanced Mobile Phone System (AMPS)

FDMA/FDD

Global System for Mobile (GSM) TDMA/FDD

Interim Standard 95 (IS-95) CDMA/FDD

W-CDMA (3GPP) CDMA/FDDCDMA/TDD

cdma2000 (3GPP2) CDMA/FDDCDMA/TDD

FDMA or TDMA?

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Used exclusively for multiple access in 1G down to individual resource units or physical channels

FDMA or TDMA?

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Transmit data in buffer-and-burstmethod

FDMA or TDMA?

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Need to use costly bandpass filters

FDMA or TDMA?

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Allows completely uncoordinated transmission in the time domain

ecs455: chapter 4 - 4 - 1 - multiple access - 1… · multiple access techniques 18 yallow many...

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